The Collaborative International Dictionary of English v.0.48:

Excuse \Ex*cuse"\, v. t. [imp. & p. p. Excused; p. pr. & vb.
   n. Excusing.] [OE. escusen, cusen, OF. escuser, excuser, F.
   excuser, fr. L. excusare; ex out + causa cause, causari to
   plead. See Cause.]
   1. To free from accusation, or the imputation of fault or
      blame; to clear from guilt; to release from a charge; to
      justify by extenuating a fault; to exculpate; to absolve;
      to acquit.
      [1913 Webster]

            A man's persuasion that a thing is duty, will not
            excuse him from guilt in practicing it, if really
            and indeed it be against Gog's law.   --Abp. Sharp.
      [1913 Webster]

   2. To pardon, as a fault; to forgive entirely, or to admit to
      be little censurable, and to overlook; as, we excuse
      irregular conduct, when extraordinary circumstances appear
      to justify it.
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            I must excuse what can not be amended. --Shak.
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   3. To regard with indulgence; to view leniently or to
      overlook; to pardon.
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            And in our own (excuse some courtly stains.)
            No whiter page than Addison remains.  --Pope.
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   4. To free from an impending obligation or duty; hence, to
      disengage; to dispense with; to release by favor; also, to
      remit by favor; not to exact; as, to excuse a forfeiture.
      [1913 Webster]

            I pray thee have me excused.          --xiv. 19.
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   5. To relieve of an imputation by apology or defense; to make
      apology for as not seriously evil; to ask pardon or
      indulgence for.
      [1913 Webster]

            Think ye that we excuse ourselves to you? --2 Cor.
                                                  xii. 19.

   Syn: To vindicate; exculpate; absolve; acquit.

   Usage: - To Pardon, Excuse, Forgive. A superior pardons
          as an act of mercy or generosity; either a superior or
          an equal excuses. A crime, great fault, or a grave
          offence, as one against law or morals, may be
          pardoned; a small fault, such as a failure in social
          or conventional obligations, slight omissions or
          neglects may be excused. Forgive relates to offenses
          against one's self, and punishment foregone; as, to
          forgive injuries or one who has injured us; to pardon
          grave offenses, crimes, and criminals; to excuse an
          act of forgetfulness, an unintentional offense. Pardon
          is also a word of courtesy employed in the sense of
          excuse.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Excuse \Ex*cuse"\, n. [Cf. F. excuse. See Excuse, v. t.]
   1. The act of excusing, apologizing, exculpating, pardoning,
      releasing, and the like; acquittal; release; absolution;
      justification; extenuation.
      [1913 Webster]

            Pleading so wisely in excuse of it.   --Shak.
      [1913 Webster]

   2. That which is offered as a reason for being excused; a
      plea offered in extenuation of a fault or irregular
      deportment; apology; as, an excuse for neglect of duty;
      excuses for delay of payment.
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            Hence with denial vain and coy excuse. --Milton.
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   3. That which excuses; that which extenuates or justifies a
      fault. "It hath the excuse of youth." --Shak.
      [1913 Webster]

            If eyes were made for seeing.
            Then beauty is its own excuse for being. --Emerson.

   Syn: See Apology.
        [1913 Webster]

WordNet (r) 3.0 (2006):

excuse
    n 1: a defense of some offensive behavior or some failure to
         keep a promise etc.; "he kept finding excuses to stay";
         "every day he had a new alibi for not getting a job"; "his
         transparent self-justification was unacceptable" [syn:
         excuse, alibi, exculpation, self-justification]
    2: a note explaining an absence; "he had to get his mother to
       write an excuse for him"
    3: a poor example; "it was an apology for a meal"; "a poor
       excuse for an automobile" [syn: apology, excuse]
    v 1: accept an excuse for; "Please excuse my dirty hands" [syn:
         excuse, pardon]
    2: grant exemption or release to; "Please excuse me from this
       class" [syn: excuse, relieve, let off, exempt]
    3: serve as a reason or cause or justification of; "Your need to
       sleep late does not excuse your late arrival at work"; "Her
       recent divorce may explain her reluctance to date again"
       [syn: excuse, explain]
    4: defend, explain, clear away, or make excuses for by
       reasoning; "rationalize the child's seemingly crazy
       behavior"; "he rationalized his lack of success" [syn:
       apologize, apologise, excuse, justify, rationalize,
       rationalise]
    5: ask for permission to be released from an engagement [syn:
       excuse, beg off]
    6: excuse, overlook, or make allowances for; be lenient with;
       "excuse someone's behavior"; "She condoned her husband's
       occasional infidelities" [syn: excuse, condone]